Bump AWS_EVENT_STREAM_MAX_HEADERS_SIZE

[graebm]

Issue:

The previous PR increased MAX_MESSAGE_SIZE from 16MiB -> 256MiB because some services were bumping into this limit. This is lower that its actual technical max of INT32_MAX (2GiB+), but it seemed like there should still have some reasonable value.

MAX_HEADERS_SIZE is a similar constant, set lower than its actual technical limit of INT32_MAX (2GiB+), and was not adjusted in the previous PR

Description of changes:

• Set MAX_HEADERS_SIZE equal to MAX_MESSAGE_SIZE
  • Yes, it's silly for a message to be 100% headers, but 🤷‍♀️
• Document how many bytes are used to encode these things, so we know which constants can be bumped higher or not.
• Add image and text to Readme (copied Transcribe docs) the explains the encoding (I always need to look this up myself)
• Remove AWS_EVENT_STREAM_HEADER_STATIC_VALUE_LEN_MAX, a value that is only used once internally, and whose existence can only serve to confuse users of this library
• Add AWS_EVENT_STREAM_HEADER_VALUE_LEN_MAX, because that is an actual technical limit a user might be interested in

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[waahm7]