Add client capabilities into the validator request for write only

[yottta]

Fixes #282.

The PreferWriteOnlyAttribute validator is misconfigured in terms that the client capabilities received in the request are not given to the validator. Therefore, any call to validate will return no error rendering the validator useless.

This PR is passing over the client capabilities to the validator.

[hashicorp-cla-app]

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[yottta]

I will read and sign the CLA next week. In case somebody that is already having this signed can apply the changes, please, go ahead.

[rainkwan]

Hi @yottta, thank you for the contribution!

In addition to signing the CLA, can you also add a changelog entry for this PR marked as BUG FIXES?

[yottta]

I will read and sign the CLA next week. In case somebody that is already having this signed can apply the changes, please, go ahead.

I didn't forget about this. I am still waiting for some information before signing the CLA.

Meanwhile, I wouldn't mind if anybody could merge the changes

[rainkwan]

Hi @yottta,

Thank you again for your contribution. It looks like this bug got fixed in this PR so I am going to go ahead and close this one.

[yottta]

Thanks a lot @rainkwan! Sorry for not being able to merge this on my own.
Then I will also close #282.