remove async_trait import

[dj8yfo]

it was noticed during review of #369 that async_trait import and annotations can be potentially removed,

more details on blog post

crate compiles with following diff and tests pass:

diff --git a/workspaces/Cargo.toml b/workspaces/Cargo.toml
index d4b1c45..0ed2f29 100644
--- a/workspaces/Cargo.toml
+++ b/workspaces/Cargo.toml
@@ -42,2 +42,3 @@ near-sandbox-utils = "0.13"
 near-chain-configs = { version = "0.28", optional = true }
+trait-variant = "0.1.2"
 
diff --git a/workspaces/src/network/betanet.rs b/workspaces/src/network/betanet.rs
index 872a01d..f6add88 100644
--- a/workspaces/src/network/betanet.rs
+++ b/workspaces/src/network/betanet.rs
@@ -28,3 +28,3 @@ pub struct Betanet {
 
-#[async_trait::async_trait]
+// #[async_trait::async_trait]
 impl FromNetworkBuilder for Betanet {
diff --git a/workspaces/src/network/builder.rs b/workspaces/src/network/builder.rs
index 2e48e44..3d7b524 100644
--- a/workspaces/src/network/builder.rs
+++ b/workspaces/src/network/builder.rs
@@ -18,4 +18,6 @@ pub(crate) type BoxFuture<'a, T> = std::pin::Pin<Box<dyn Future<Output = T> + Se
 /// ```
-#[async_trait::async_trait]
-pub(crate) trait FromNetworkBuilder: Sized {
+// #[async_trait::async_trait]
+
+#[trait_variant::make(FromNetworkBuilder: Send)]
+pub(crate) trait LocalFromNetworkBuilder: Sized {
     async fn from_builder<'a>(build: NetworkBuilder<'a, Self>) -> crate::result::Result<Self>;
diff --git a/workspaces/src/network/custom.rs b/workspaces/src/network/custom.rs
index c47296a..3a67992 100644
--- a/workspaces/src/network/custom.rs
+++ b/workspaces/src/network/custom.rs
@@ -13,3 +13,3 @@ pub struct Custom {
 
-#[async_trait::async_trait]
+// #[async_trait::async_trait]
 impl FromNetworkBuilder for Custom {
diff --git a/workspaces/src/network/mainnet.rs b/workspaces/src/network/mainnet.rs
index 65ab757..a94e044 100644
--- a/workspaces/src/network/mainnet.rs
+++ b/workspaces/src/network/mainnet.rs
@@ -29,3 +29,3 @@ pub struct Mainnet {
 
-#[async_trait::async_trait]
+// #[async_trait::async_trait]
 impl FromNetworkBuilder for Mainnet {
diff --git a/workspaces/src/network/sandbox.rs b/workspaces/src/network/sandbox.rs
index 9c97526..e7a328d 100644
--- a/workspaces/src/network/sandbox.rs
+++ b/workspaces/src/network/sandbox.rs
@@ -125,3 +125,3 @@ impl std::fmt::Debug for Sandbox {
 
-#[async_trait]
+// #[async_trait]
 impl FromNetworkBuilder for Sandbox {
@@ -132,3 +132,3 @@ impl FromNetworkBuilder for Sandbox {
 
-#[async_trait]
+// #[async_trait]
 impl TopLevelAccountCreator for Sandbox {
@@ -178,3 +178,3 @@ impl TopLevelAccountCreator for Sandbox {
 
-#[async_trait]
+// #[async_trait]
 impl SponsoredAccountCreator for Sandbox {
diff --git a/workspaces/src/network/testnet.rs b/workspaces/src/network/testnet.rs
index 7f6efd2..8721675 100644
--- a/workspaces/src/network/testnet.rs
+++ b/workspaces/src/network/testnet.rs
@@ -40,3 +40,3 @@ pub struct Testnet {
 
-#[async_trait]
+// #[async_trait]
 impl FromNetworkBuilder for Testnet {
@@ -68,3 +68,3 @@ impl std::fmt::Debug for Testnet {
 
-#[async_trait]
+// #[async_trait]
 impl SponsoredAccountCreator for Testnet {
diff --git a/workspaces/src/network/variants.rs b/workspaces/src/network/variants.rs
index e12c749..96e4952 100644
--- a/workspaces/src/network/variants.rs
+++ b/workspaces/src/network/variants.rs
@@ -25,3 +25,3 @@ pub trait NetworkInfo {
 /// It is expected that the `subaccount_prefix` does not contain a `.`.
-#[async_trait]
+// #[async_trait]
 pub trait SponsoredAccountCreator {
@@ -43,3 +43,3 @@ pub trait SponsoredAccountCreator {
 
-#[async_trait]
+// #[async_trait]
 pub trait TopLevelAccountCreator {