feat: use int64 type for claims with timestamps

[nestorvw]

Proposed changes

Uses int64 for timestamped claim values.

Checklist

• I have read the contributing guidelines
  and signed the CLA.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added necessary documentation within the code base (if
  appropriate).

[narg95]

@aeneasr and @mitar: This PR transform claims with timestamps to int64 data type
Marshalling to a JWT token works out-of-the-box with go-jose, but unmarshalling does not.
Therefore I opened issue #351 in go-jose and in the meantime I workarounded it with a simple float64->int64 conversion on first level claims.

[mitar]

I like it!

[aeneasr]

Could you please take a look at the failing test cases? Thanks! :)

[narg95]

fixed

[aeneasr]

Great job as always @narg95 - thank you!

[narg95]

@aeneasr my pleasure :)