Testing Plan - Send arrays of events through the topology (#11072)

Context

As this PR introduces EventArray, it represents a change to the type flowing through buffers. In order to prevent a backwards-incompatible change, this PR adds a change that allows the encoding scheme to try to decode as either EventArray or Event in order to be able to load older buffer files and continue processing them while using EventArray going forward.

This test ensures that we can write Events to the buffer, and then read them out when using the newer code.

Plan

Start by grabbing a Vector binary for 0.19.0, and build one from the PR branch. We'll use a simple configuration that will read records from stdin and attempt to send them to an HTTP sink.

For the HTTP sink, we'll have one configuration that uses an invalid port and another, an identical version, which has the correct port. Since Vector will retry "connection refused" errors, and retry them infinitely, the messages will never be acknowledged, which ensures they remain in the buffer. For the purposes of verifying that the same data that went in is still present after renaming the data directory, etc, we can use dummyhttp (a Rust project for serving up a simplistic HTTP endpoint that can be configured to respond a certain way) to listen on the port and inspect the HTTP request made by the sink.

Test Case(s)

1. Ensure that records written to a v1 disk buffer from a version of Vector without this change can be read back from a version of Vector with this change:

   • Start dummyhttp listening on the relevant port.
   • Run the 0.19.0 binary, using the "wrong" configuration, with a clean data directory. The five-lines-first file should be piped to STDIN.
   • Stop Vector.
   • Run the PR binary, using the "right" configuration, and ensure that it reads the records from the buffer and sends them to the HTTP sink. This should be five records: all five from the run using Vector 0.19.0.

2. Ensure that records written to a v1 disk buffer from a version of Vector without this change can be read back from a version of Vector with this change, even after additionally writing events to the buffer that come from a version of Vector with this change:

   • Start dummyhttp listening on the relevant port.
   • Run the 0.19.0 binary, using the "wrong" configuration, with a clean data directory. The five-lines-first file should be piped to STDIN.
   • Stop Vector.
   • Run the PR binary, using the "wrong" configuration. The five-lines-second file should be piped to STDIN.
   • Stop Vector.
   • Run the PR binary, using the "right" configuration, and ensure that it reads all the records from the buffer and sends them to the HTTP sink. This should be ten records: all five from five-lines-first, and all five from five-lines-second.