Make the package safe for concurrent access

[alovak]

With this PR, we've enhanced the concurrency safety of the methods in iso8583.Message and field.Composite.

Rationale

I encountered a data race when two goroutines executed message.GetString(11). This happened because the GetString method did more than just returning a value — it also updated an internal map. This concurrent access led to the race condition.

Assumptions on Concurrency:

• Individual field access (like field.String, field.Numeric) is not thread-safe. Concurrency safety is ensured at the Message and Composite levels. If external concurrent access to individual fields is necessary, users should manage it with appropriate synchronization mechanisms, such as a mutex.
• Concurrent access is expected to be infrequent. Given this, the overhead introduced by using a mutex within Message and Composite is likely to be minimal.

Concerns

A potential solution to the data race I encountered would have been to simply eliminate the unnecessary map modification, thus making GetXXX() methods pure getters (which is also done in the PR). However, because the message is referenced via a pointer and has a combination of getters, setters, Marshal, and Unmarshal methods, it doesn't feel entirely safe to assume that access to these will always be sequential. So, enhancing concurrency safety seems like a guarded measure.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (b652df4) 73.70% compared to head (d2157a7) 73.95%.

❗ Current head d2157a7 differs from pull request most recent head a0dcd00. Consider uploading reports for the commit a0dcd00 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
+ Coverage   73.70%   73.95%   +0.24%     
==========================================
  Files          43       43              
  Lines        2282     2338      +56     
==========================================
+ Hits         1682     1729      +47     
- Misses        369      378       +9     
  Partials      231      231              

Files	Coverage Δ
test/fuzz-reader/reader.go	55.55% <0.00%> (ø)
message.go	74.90% <91.83%> (+2.95%)	⬆️
field/composite.go	80.79% <85.36%> (-1.09%)	⬇️

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