[receiver/netflowreceiver] add send_raw option for sending unparsed logs (open-telemetry#38832)

#### Description
- Add a `send_raw` option to the config. 
- When set to true, the NetFlow log messages will be sent as an unparsed
string in the log body.
- When send_raw is false or is not configured, logs are parsed into
attributes as before.
- This works for all kinds of NetFlow log data, such as v5, v9, etc.

#### Link to tracking issue
Closes:
open-telemetry#38920

#### Testing
- New test in producer_test.go
- Update test in config_test.go
- Testing sending logs to Google SecOps:
- Simulate NetFlow logs:
https://hub.docker.com/r/networkstatic/nflow-generator
    - Set up configuration.yaml sending Netflow to Google SecOps
    - Confirm output of unparsed logs in Google SecOps

#### Documentation
- Update README.md with documentation of the `send_raw` field

Author: justinianvoss22

.chloggen/netflowreceiver-add-send-raw-option.yaml

@@ -0,0 +1,25 @@
+# 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: netflowreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add `send_raw` option to send logs as a raw string in the log body instead of parsed into attributes.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [38920]
+
+# (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]