load_balancing: support header session affinity policy

[ColeBennett]

Adds support for the new header session affinity policy and its related session affinity attribute.

Has your change been tested?

Tested the API using curl and the SDK locally with a Go client.

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Types of changes

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas and relies on stable APIs.

[github-actions]

changelog detected ✅

[codecov-commenter]

Codecov Report

Merging #1302 (d4d50c9) into master (b9ac804) will increase coverage by 0.08%.
The diff coverage is 58.14%.

@@            Coverage Diff             @@
##           master    #1302      +/-   ##
==========================================
+ Coverage   48.33%   48.42%   +0.08%     
==========================================
  Files         133      134       +1     
  Lines       13023    13104      +81     
==========================================
+ Hits         6295     6345      +50     
- Misses       5201     5220      +19     
- Partials     1527     1539      +12     

Impacted Files	Coverage Δ
load_balancing.go	59.40% <ø> (ø)
regional_tiered_cache.go	43.75% <43.75%> (ø)
logpush.go	51.81% <46.15%> (-0.30%)	⬇️
images.go	44.70% <47.82%> (-0.12%)	⬇️
rulesets.go	30.03% <52.17%> (-4.61%)	⬇️
pages_deployment.go	38.88% <61.76%> (+4.64%)	⬆️
pages_project.go	58.22% <90.00%> (+7.40%)	⬆️
access_policy.go	70.27% <100.00%> (ø)
pagination.go	100.00% <100.00%> (+28.57%)	⬆️

[github-actions]

This functionality has been released in v0.73.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!