ShallowEtagHeaderFilter use specified contentlength

Prior to this commit, the ShallowEtagHeaderFilter did not use the
content length given by the content generator to set the
ByteArrayOutputStream's buffer size.
This can lead to performance issues for large content since the buffer
grows as the content is being written.

This commit adds a new ByteArrayOutputStream variant called
ResizableByteArrayOutputStream. This implementation has public methods
for modifying the internal buffer size and does not synchronize on
buffer access.
This commit also make use of this new variant in
ShallowEtagHeaderFilter.

Issue: SPR-8271