Add Billing Meters and Billing Meter Events

[winsmith]

This PR implements these APIs:

• Billing Meters v1
• Billing Meter Events v1

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 115 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@5c579be). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
Sources/StripeKit/Billing/Meters/MeterRoutes.swift	0.00%	59 Missing ⚠️
...ipeKit/Billing/Meter Events/MeterEventRoutes.swift	0.00%	21 Missing ⚠️
Sources/StripeKit/StripeClient.swift	0.00%	17 Missing ⚠️
Sources/StripeKit/Products/Prices/Price.swift	0.00%	10 Missing ⚠️
...es/StripeKit/Billing/Meter Events/MeterEvent.swift	0.00%	8 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             main    #275   +/-   ##
======================================
  Coverage        ?   2.02%           
======================================
  Files           ?     197           
  Lines           ?   10380           
  Branches        ?       0           
======================================
  Hits            ?     210           
  Misses          ?   10170           
  Partials        ?       0           

Files with missing lines	Coverage Δ
...es/StripeKit/Billing/Meter Events/MeterEvent.swift	0.00% <0.00%> (ø)
Sources/StripeKit/Products/Prices/Price.swift	0.00% <0.00%> (ø)
Sources/StripeKit/StripeClient.swift	0.00% <0.00%> (ø)
...ipeKit/Billing/Meter Events/MeterEventRoutes.swift	0.00% <0.00%> (ø)
Sources/StripeKit/Billing/Meters/MeterRoutes.swift	0.00% <0.00%> (ø)