Add CA bundle version and pinning status to the user agent string

[cisco-dmytro-hissa]

Description

• Added a caBundleVersion = "1.0" constant.
• Added a userAgent() method on *Client that builds the User-Agent string and appends ca_bundle/1.0 plus (ca_pinning=enabled|disabled) based on the client's caPinning field.
• Replaced the inline fmt.Sprintf User-Agent construction at the token request call site with a call to client.userAgent().

How Has This Been Tested?

• Updated TestSendUserAgent to expect the new User-Agent format including ca_bundle and (ca_pinning=enabled).
• Added TestUserAgentCAPinningEnabled — verifies the string ends with (ca_pinning=enabled) when pinning is on.
• Added TestUserAgentCAPinningDisabled — verifies the string ends with (ca_pinning=disabled) when pinning is off.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)