ClientRegistration.Builder.tokenUri() should take a `URI` parameter, not a `String`

[walles]

Expected Behavior

ClientRegistration.Builder.tokenUri() accepts a URI as its parameter.

		public Builder tokenUri(URI tokenUri) {  // <- Note the data type of the parameter
			this.tokenUri = tokenUri;
			return this;
		}

Current Behavior

ClientRegistration.Builder.tokenUri() accepts 🚨 a String as its tokenUri parameter.

spring-security/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/ClientRegistration.java

Lines 527 to 530 in 0137d02

	public Builder tokenUri(String tokenUri) {
	this.tokenUri = tokenUri;
	return this;
	}

Context

We accidentally sent the wrong parameter to ClientRegistration.Builder.tokenUri().

This cost us troubleshooting time.

If tokenUri() had required a URI rather than a String, the compiler would have caught this, saving us troubleshooting time.

Note that this would be an API breaking change, so I'm guessing this fix would have to wait for v7.

Note also that some related fields / methods should also get their types fixed accordingly. At least in this file, but potentially elsewhere in the project as well.

[walles]

Unsure whether this should go under "Bug" or "Enhancement". I sort of feel this is a bug, but in the interest of not crying wolf I settled for Enhancement.