Support validation for the length of auth0_client description length

[tanmng]

Proposed Changes

• Introduce ValidateFunc to the description field of auth0_client resource.

Currently, since we don't have a validation for this field, users can run into issue when they try to create/update an auth0_client where the description field is too long. The error from Auth0 is:

Error: 400 Bad Request: Payload validation error: 'String is too long (xxx chars), maximum 140' on property description (Free text description of the purpose of the Client. (Max character length: <code>140</code>)).

Acceptance Test Output

$ make testacc TESTS=TestAccXXX

==> Checking that code complies with gofmt requirements...
?   	github.com/terraform-providers/terraform-provider-auth0	[no test files]
=== RUN   TestAccClientGrant
--- PASS: TestAccClientGrant (47.05s)
=== RUN   TestAccClient
--- PASS: TestAccClient (2.65s)
=== RUN   TestAccClientZeroValueCheck
--- PASS: TestAccClientZeroValueCheck (6.19s)
=== RUN   TestAccClientRotateSecret
--- PASS: TestAccClientRotateSecret (4.48s)
=== RUN   TestAccClientInitiateLoginUri
--- PASS: TestAccClientInitiateLoginUri (0.03s)
=== RUN   TestAccClientJwtScopes
--- PASS: TestAccClientJwtScopes (4.48s)
PASS
coverage: 25.3% of statements
ok  	github.com/terraform-providers/terraform-provider-auth0/auth0	64.987s	coverage: 25.3% of statements
?   	github.com/terraform-providers/terraform-provider-auth0/auth0/internal/debug	[no test files]
testing: warning: no tests to run
PASS
coverage: 0.0% of statements
ok  	github.com/terraform-providers/terraform-provider-auth0/auth0/internal/random	0.072s	coverage: 0.0% of statements [no tests to run]
testing: warning: no tests to run
PASS
coverage: 0.0% of statements
ok  	github.com/terraform-providers/terraform-provider-auth0/auth0/internal/validation	0.020s	coverage: 0.0% of statements [no tests to run]
?   	github.com/terraform-providers/terraform-provider-auth0/version	[no test files]

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[alexkappa]

Thank you @tanmng!