GH-764: Add signing keys data source

[sergiught]

🔧 Changes

As requested in #764, we're adding a data source for retrieving application signing keys.

The usage will look similar to:

data "auth0_signing_keys" "my_keys" {}

# Example on how to get the current key from the data source.
output "current_key" {
  value = try(
    element([for key in data.auth0_signing_keys.my_keys.signing_keys : key.kid if key.current], 0),
    "No current key found"
  )
}

📚 References

• Return signing key ID in outputs of auth0_client #764

🔬 Testing

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

Merging #839 (8cf7943) into v1 (fd294eb) will increase coverage by 0.04%.
The diff coverage is 96.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##               v1     #839      +/-   ##
==========================================
+ Coverage   89.85%   89.89%   +0.04%     
==========================================
  Files          99      101       +2     
  Lines       13696    13783      +87     
==========================================
+ Hits        12306    12390      +84     
- Misses        987      989       +2     
- Partials      403      404       +1     

Files Changed	Coverage Δ
internal/provider/provider.go	100.00% <ø> (ø)
internal/auth0/signingkey/data_source.go	95.77% <95.77%> (ø)
internal/auth0/signingkey/flatten.go	100.00% <100.00%> (ø)