Add Target for Bedrock Flows

[athewsey]

Issue #, if available: Closes #80

Description of changes:

This PR adds a basic agenteval Target for testing Bedrock Flows. Key caveats include:

1. As far as I'm aware, Flows are stateless and this PR doesn't attempt to do anything about that on agenteval side: So it's mainly expecting single-turn tests
2. I was surprised to see the existing BedrockAgentTarget doesn't seem to actually do anything when it receives error chunks in the response stream... I wrote the flow target to treat these as an error and raise them for now, but maybe different behaviour is expected?
3. While the InvokeFlow API seems to imply all sorts of interesting things like multiple inputs and multiple outputs... As far as I can tell at the moment (from building flows in the console) there's always exactly one input called FlowInputNode. This sits pretty nicely anyway with agenteval's "string prompt in, string response out" assumptions.

Testing & docs checklist:

• Added a (basic) unit test for the new target
• Checked the overall agenteval run works with a simple Flow & couple of single-turn scripts in my account
• Updated the "targets" section of the docs site

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[sharonxiaohanli]

LGTM!