Supply more useful variables to @Encrypted keyId SPEL expression

The keyId SPEL expression for the @Encrypted annotation is great for avoiding hard-coding of encryption key ids. See docs here. Example:

@Document
@Encrypted(keyId = "#{mongocrypt.keyId(#target)}")
static class Patient {

    @Id String id;
    String name;

    @Encrypted(algorithm = "AEAD_AES_256_CBC_HMAC_SHA_512-Random")
    String bloodType;

    @Encrypted(algorithm = "AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic")
    Integer ssn;
}

However the value bound to #target isn't particularly useful, it's just the simple class name (minus the package!) of the annotated class. See this line:

ctx.setVariable("target", getType().getSimpleName());

This limits the usefulness of the SPEL expression, especially in scenarios where users might want to use different encryption keys for different collections.

A simple but impactful enhancement would be to bind collection as a variable:

ctx.setVariable("collection", getCollection());

Curious to hear people's thoughts on this.

Start at BasicMongoPersistentEntity.java around the shown ctx.setVariable("target", getType().getSimpleName()) line and inspect how getCollection() is available there. Bind the collection name as an additional SPEL variable, then verify that @Encrypted keyId expressions can use it for collection-specific key selection.