Draft of a functional VaultClient

[mp911de]

Do not merge.

Rename VaultClient to PreviousVaultClient. Introduce VaultClient and VaultClientRequest interfaces. Add default implementations with DefaultVaultClient and DefaultVaultClientRequest. Introduce VaultRequestBody as body provider.

Usage:

VaultResponseEntity<String> response = vaultClient.get()
				.uri("/{hello}/{world}", "hello", "world")
				.header("X-Vault-Token", "…")
				.exchange(String.class);

VaultResponseEntity<Void> response = vaultClient.get()
				.uri("/{hello}/{world}", "hello", "world")
				.header("X-Vault-Token", "…")
				.body(VaultRequestBody.just("{ \"ok\": 1}"))
				.exchange();

VaultResponseEntity<Void> response = vaultClient.get()
				.uri("/example")
				.exchange();

VaultClient defines a builder-style API to build requests with exposing only contextual API for the current build step

[mp911de]

See also #49.

[mp911de]

Agreed to remove VaultClient entirely.