Add option for validating content-type header

For systems implementing to the OWASP security guidelines[1] (of which all
should ideally) we should be checking the content-type header all times
that we parse a request as JSON.

This commit adds an option for parsing content-type, and sets a default
of "must get content-type".  Ideally this would not be a breaking
change, but given the number of guides and scripts that omit the content
type, it seems worthwhile to add a trapdoor, such that people can opt
into their own model on how they would like to see this checking work.

Tested:
```
curl --insecure -H "Content-Type: application/json" -X POST -D headers.txt https://${bmc}/redfish/v1/SessionService/Sessions -d    '{"UserName":"root", "Password":"0penBmc"}'
```

Succeeds.

Removing Content-Type argument causes bmc to return
Base.1.13.0.UnrecognizedRequestBody.

[1] cheatsheetseries.owasp.org/cheatsheets/REST_Security_Cheat_Sheet.html

Change-Id: Iaa47dd563b40036ff2fc2cacb70d941fd8853038
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/meson.build b/meson.build
index be3c6b1..ae789a9 100644
--- a/meson.build
+++ b/meson.build
@@ -70,6 +70,7 @@
   'insecure-disable-ssl'                        : '-DBMCWEB_INSECURE_DISABLE_SSL',
   'insecure-push-style-notification'            : '-DBMCWEB_INSECURE_ENABLE_HTTP_PUSH_STYLE_EVENTING',
   'insecure-tftp-update'                        : '-DBMCWEB_INSECURE_ENABLE_REDFISH_FW_TFTP_UPDATE',
+  'insecure-ignore-content-type'                : '-DBMCWEB_INSECURE_IGNORE_CONTENT_TYPE',
   'kvm'                                         : '-DBMCWEB_ENABLE_KVM' ,
   'mutual-tls-auth'                             : '-DBMCWEB_ENABLE_MUTUAL_TLS_AUTHENTICATION',
   'redfish-aggregation'                         : '-DBMCWEB_ENABLE_REDFISH_AGGREGATION',