Add Portal handler D-Bus interface

[iinuwa]

This adds the necessary changes for xdg-desktop-portal to initiate the request to credentialsd. Once this is ready, this will become the primary interface for interacting with credentialsd.

Unfortunately, this is a huge PR. Some highlights of changes:

• Adds the portal interface
• Uses HTTPS origins for both privileged and unprivileged apps. (access control will be implemented in XDP using entitlements once that lands.)
• Because of the above, origin is now required to be set.
• top_origin can now be sent in options.json.
• Rearranges some fields, most notably, flattening out the request_json fields in the request, which should make construction of the request a bit simpler.
• Updated the api.md doc to reflect the portal gateway interface, since that is becoming the primary gateway interface.
• Defines options.activation_token optional parameter, but credentialsd doesn't do anything with it yet.

In order to test this, you need to run iinuwa/xdg-desktop-portal@65f21d3 with the XDG_DESKTOP_PORTAL_ENABLE_EXPERIMENTAL=credential environment variable set, as well as having the impl portal stub defined in ~/.local/share/xdg-desktop-portal/portals/credentialsd.portal

[portal]
DBusName=xyz.iinuwa.credentialsd.Credentials
Interfaces=org.freedesktop.impl.portal.Credential
UseIn=sway # or whatever DE you use

and having ~/.config/xdg-desktop-portal/<desktop-environment>.config populated with

[preferred]
org.freedesktop.impl.portal.Credential=credentialsd

This should allow xdg-desktop-portal to start the Credential portal at org.freedesktop.portal.experimental.Credential. (You can use D-Spy, busctl, or some other introspection to verify whether the interface was started.)