Allow users to configure initial buffer size (open-telemetry#37786)

Using the filelogreceiver with a system that does not allow for
in-progress file reads (holds a lock on the file while writing) and we
want to read the entire file (can think of a big proto binary) then
decode and construct logs.

Because of this use-case and because of the files being ~5MB we do a lot
of re-size of the scanner buffer until we are able to read the whole
file/token.

Proposal is to allow customizing the initial buffer size for use-cases
like this.

---------

Signed-off-by: Bogdan Drutu <[email protected]>

.chloggen/add-initial-buffer.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: pkg/stanza
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Allow users to configure initial buffer size
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37786]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]