feat: set User-Agent header in gRPC.

[karel-rehor]

Proposed Changes

1. Move setting UserAgent value to AssemblyHelper
2. Ensure the header is added to Metadata in FlightSqlClient
3. Add check for user-agent header in tests
4. Ensure that only 1 user-agent header can be defined per call, and that it is the default from AssemblyHelper

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.73%. Comparing base (b437cb0) to head (8065285).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   95.70%   95.73%   +0.02%     
==========================================
  Files          12       12              
  Lines        1072     1079       +7     
  Branches      136      137       +1     
==========================================
+ Hits         1026     1033       +7     
  Misses         11       11              
  Partials       35       35              

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[bednar]

Thanks for your PR 👍

LGTM 🚀