Support adding certificates to trust store by specifying URL

[FeynmanZhou]

What is the areas you would like to add the new feature to?

Notation CLI

Is your feature request related to a problem?

In general, users need to download a public key from a remote server and add the public key file to the trust store using notation cert add when verifying the signature. It's a little cumbersome during this process.

If there is an option to allow the verifier to load the public key to the trust store by specifying the URL when using notation cert add, it would be much more convenient for users in the signature verification process.

What solution do you propose?

For example, specifying the public certificate through an HTTPS address when using notation cert add as follows:

$ notation cert add --type ca --store acme-rockets https://www.helloworld.com/pkiops/certs/Root-CA-2023.crt
Successfully added the following certificate to named store acme-rockets of type ca:
Root-CA-2023.crt

What alternatives have you considered?

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Any additional context?

No response