feat(postgrest): immutability

[Vinzent03]

Changes

• Makes the postgrest query building immutable
  Makes the following possible

final query = postgrest.from("table").select();
final res1 = await query.eq("id", 1);
final res2 = await query.gt("id", 2);

• BREAKING: make PostgrestQueryBuilder.url a required named parameter
• BREAKING: remove PostgrestBuilder.execute()

[dshukertjr]

This looks awesome!

Could we also add the @immutable annotation to every class that should be immutable?

[dshukertjr]

On a second thought, maybe we can just do all the "make bunch of things private" in a separate PR.

[Vinzent03]

I've added @immutable only to PostgrestBuilder, because all classes extending that class are immutable as well from the docs:

Used to annotate a class C. Indicates that C and all subtypes of C must be immutable.