Allow for User API tokens to be used with DNSimple

[IntegralProgrammer]

What would you like to be added:

The ability to use User API tokens as opposed to only Account API tokens for interacting with the DNSimple API. Given that a User API token does not map to one specific account, the account ID would need to be specified via an environment variable (eg. DNSIMPLE_ACCOUNT_ID). Given that User API tokens do not allow for listing the DNS Zones of another DNSimple account (even if this user has access permissions to some of these zones), the DNS zones which External-DNS manipulates would need to be specified via environment variable (eg. DNSIMPLE_ZONES).

Why is this needed:

If the owner of a DNSimple account only wishes to give edit permissions to certain domains in that account, said owner could assign these permissions to a specific user and therefore a User API token and not an Account API token would need to be used for programmatically changing DNS records.

I have written a Pull Request to implement this feature - #4274.