Add JWT support

[JU4N98]

Changes proposal:

These changes allow JWT svids and JWK bundles getting by specifying an audience, JWT file name and JWK file name.

fixes #43

[keeganwitt]

config.go is still requiring svid_file_name and related values be set, which aren't necessary when just using the JWT functionality.

[faisal-memon]

config.go is still requiring svid_file_name and related values be set, which aren't necessary when just using the JWT functionality.

Good catch @keeganwitt

[JU4N98]

config.go is still requiring svid_file_name and related values be set, which aren't necessary when just using the JWT functionality.

Good catch @keeganwitt

Should we validate that x509, JWT or JWT bundles are going to be fetched? Or simply delete this validation?

[keeganwitt]

config.go is still requiring svid_file_name and related values be set, which aren't necessary when just using the JWT functionality.

Good catch @keeganwitt

Should we validate that x509, JWT or JWT bundles are going to be fetched? Or simply delete this validation?

I was thinking

if c.SvidFileName == "" && c.JWTFilename == "" {
    return errors.New("svid_file_name and/or jwt_file_name is required")
}
if c.SvidFileName != "" && c.SvidKeyFileName == "" {
    return errors.New("svid_key_file_name is required when using svid_file_name")
}
if c.SvidFileName != "" && c.SvidBundleFileName == "" {
    return errors.New("svid_bundle_file_name is required when using svid_file_name")
}
if c.JWTFilename != "" && c.JWKFilename == "" {
    return errors.New("jwk_file_name is required when using jwt_file_name")
}

[JU4N98]

config.go is still requiring svid_file_name and related values be set, which aren't necessary when just using the JWT functionality.

Good catch @keeganwitt

Should we validate that x509, JWT or JWT bundles are going to be fetched? Or simply delete this validation?

I was thinking

if c.SvidFileName == "" && c.JWTFilename == "" {
    return errors.New("svid_file_name and/or jwt_file_name is required")
}
if c.SvidFileName != "" && c.SvidKeyFileName == "" {
    return errors.New("svid_key_file_name is required when using svid_file_name")
}
if c.SvidFileName != "" && c.SvidBundleFileName == "" {
    return errors.New("svid_bundle_file_name is required when using svid_file_name")
}
if c.JWTFilename != "" && c.JWKFilename == "" {
    return errors.New("jwk_file_name is required when using jwt_file_name")
}

Well I think there are three options, fetch X509 SVIDs, fetch JWT SVID or fetch JWT Bundle. So first and fourth conditions should be:

if c.SvidFileName == "" && c.JWTFIlename == "" && c.JWKFilename == "" {
// error message
}
if c.JWTFileName != "" && c.JWTAudience == "" {
// error message
}

Besides of that, in util_posix.go and util_windows.go, if some of this conditions aren't met no routines will be running. So maybe is better to delete these validations.

[JU4N98]

@JU4N98 There are 5 open comments left. Please let us know when you have updated.

I think is ready for a last review @faisal-memon , @maxlambrecht and @MarcosDY

[faisal-memon]

Thanks for making the request changes @JU4N98 . This looks good to me. @MarcosDY @maxlambrecht are we good to merge?

[maxlambrecht]

Thanks, @JU4N98