check for the max length in BerTLV prefix

[alovak]

In the spec I'm working with now, there is a max length for BerTLV fields.

Currently, we are ignoring the Length (max length) of the field spec for prefix.BerTLV. This PR changes the prefix a little bit:

• if Length is set to 0 or is not set (0 is zero value used for the Length anyway) - we do not check max length - this is to support current integrations which, most probably, follow the example from our tests like this:

				Subfields: map[string]field.Field{
					"9A": field.NewString(&field.Spec{
						Description: "Transaction Date",
						Enc:         encoding.Binary,
						Pref:        prefix.BerTLV,
					}),
					"9F02": field.NewString(&field.Spec{
						Description: "Amount, Authorized (Numeric)",
						Enc:         encoding.Binary,
						Pref:        prefix.BerTLV,
					}),
				},

• if Length is set to non-zero value, perform the check

This PR is a breaking change (potentially) if you use the prefix.BerTLV and set non-zero Length in the field spec.

[codecov-commenter]

Codecov Report

Patch coverage: 75.00% and project coverage change: +0.05% 🎉

Comparison is base (026d31c) 73.43% compared to head (93bf5ae) 73.49%.
Report is 1 commits behind head on master.

❗ Current head 93bf5ae differs from pull request most recent head 105736b. Consider uploading reports for the commit 105736b to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   73.43%   73.49%   +0.05%     
==========================================
  Files          43       43              
  Lines        2270     2290      +20     
==========================================
+ Hits         1667     1683      +16     
- Misses        373      375       +2     
- Partials      230      232       +2     

Files Changed	Coverage Δ
field/composite.go	83.22% <60.00%> (ø)
field/bitmap.go	71.05% <75.00%> (+0.29%)	⬆️
prefix/bertlv.go	87.87% <83.33%> (-4.13%)	⬇️
message.go	71.30% <100.00%> (+0.52%)	⬆️

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