[confignet] Change `Transport` from `string` to `TransportType`

[TylerHelmuth]

Description: Changes Transport from a string to a new TransportType. Implements UnmarshalText for TransportType to enforce values.

This PR may be too much - it introduces a breaking change a lot of new public APIs that may not be worth it for such a small module. If we don't like the surface area this creates or the breaking change, but we still want to enforce transport type values, I think implementing Validate keeps the API footprint smaller and isn't breaking.

Link to tracking Issue: <Issue number if applicable>

Closes https://github.com/open-telemetry/opentelemetry-collector/issues/9364

Documentation: <Describe the documentation added.>

Added godoc comments

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.97%. Comparing base (cc485e0) to head (21608ea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9385   +/-   ##
=======================================
  Coverage   90.96%   90.97%           
=======================================
  Files         353      353           
  Lines       18626    18640   +14     
=======================================
+ Hits        16943    16957   +14     
  Misses       1356     1356           
  Partials      327      327           

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[github-actions[bot]]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[mx-psi]

@bogdandrutu This is waiting on your review

[TylerHelmuth]

All tests are now passing except Contrib, which I'll fixed after this is merged should we choose to go forward with this breaking change approach.