Expand allowed_domains policy to support email regexes

[jaywink]

In EMS we allow customers to define a list of email regexes for who can register to a homeserver. Often these are full domains but also often individual emails. Supporting regexes would be the easiest way to cover both use cases.

Code ref for current allowed_domains check:

matrix-authentication-service/policies/email.rego

Line 20 in a62aa87

email_domain_allowed {

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#1870