* REST KmsConnector implementation
Description
diff-1: Address review comments.
Add utility interface to Platform namespace to
create and operate on tmpfile
diff-2: Address review comments
Link Boost::filesystem to CMake build process
Major changes includes:
1. Implement REST based KmsConnector implementation.
2. Salient features of the connector:
2.1. Two required configuration are:
a. Discovery KMS URLs - enable KMS discovery on bootstrap
b. Endpoint path configuration to construct URI to fetch/refresh
encryption keys
c. Configuration to provide "validationTokens" to connect with
external KMS. Patch implements file-based token validation scheme.
2.2. On startup, RESTKmsConnector discovers KMS Urls and caches
them in-memory. Extracts "validationTokens" based on input config.
2.3. Expose endpoints to allow fetch/refresh of encryption keys.
2.4. Defines JSON format to interact with external KMS - request &
response payload format.
3. Extend Platform namespace with an interface to create and operate on
tmp files.
4. Update Platform 'readFileBytes' and 'writeFileBytes' to leverage
fstream supported implementation.
NOTE: KMS URLs fetched after initial discovery will be persisted using
DynamicKnobs. It is TODO at the moment and shall be completed
once DynamicKnobs is feature complete
Testing
Unit test to validation following:
1. Parsing on "validation tokens" logic.
2. Construction and parsing of REST JSON request and response strings.
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