Add use support for OAuth app inline JWKS

[laurynas-jonusas]

Summary

Adds use support to the okta_app_oauth.jwks block. This fixes an issue where inline signing keys used with private_key_jwt would lose use = "sig|enc" and set it to null which would break the key after an OAuth app update because the provider did not expose or send the field.

Changes

• Adds jwks.use with valid values sig and enc.
• Sends use = "sig" for RSA/EC signing keys.
• Sends use = "enc" for RSA encryption keys.
• Reads use from Okta API responses back into Terraform state.
• Normalizes missing/null signing use to sig.
• Preserves configured JWKS order by matching keys by kid to avoid TypeList positional drift when Okta returns keys in a different order.
• Updates docs and examples.

Testing

• go test ./okta/services/idaas
• Manual local-provider test with terraform apply against Okta:
  • created OAuth service app with inline RSA JWKS and use = "sig"
  • added another generated RSA JWKS key
  • removed one key and verified final planned after.jwks
  • verified Okta API response kept use = "sig"

Real-infra acceptance test:

TF_ACC=1 go test ./okta/services/idaas \
  -v -run '^TestAccResourceOktaAppOauth_jwksUsePreservedWhenAddingKey$' \
  -timeout 10m