[SVLS-7704] add support for SSM Parameter API key (#924)

## Overview
* Add support for customers storing Datadog API Key in SSM Parameter
Store.

## Testing
* Deployed changes and confirmed this work with Parameter Store String
and SecureString.

Author: jchrostek-dd

env.rs

@@ -357,6 +357,11 @@ pub struct EnvConfig {
     /// The AWS KMS API key to use for the Datadog Agent.
     #[serde(deserialize_with = "deserialize_optional_string")]
     pub kms_api_key: Option<String>,
+    /// @env `DD_API_KEY_SSM_ARN`
+    ///
+    /// The AWS Systems Manager Parameter Store parameter ARN containing the Datadog API key.
+    #[serde(deserialize_with = "deserialize_optional_string")]
+    pub api_key_ssm_arn: Option<String>,
     /// @env `DD_SERVERLESS_LOGS_ENABLED`
     ///
     /// Enable logs for AWS Lambda. Default is `true`.
@@ -609,6 +614,7 @@ fn merge_config(config: &mut Config, env_config: &EnvConfig) {
     // AWS Lambda
     merge_string!(config, env_config, api_key_secret_arn);
     merge_string!(config, env_config, kms_api_key);
+    merge_string!(config, env_config, api_key_ssm_arn);
     merge_option_to_value!(config, env_config, serverless_logs_enabled);
     merge_option_to_value!(config, env_config, serverless_flush_strategy);
     merge_option_to_value!(config, env_config, enhanced_metrics);
@@ -952,6 +958,7 @@ mod tests {
                 api_key_secret_arn: "arn:aws:secretsmanager:region:account:secret:datadog-api-key"
                     .to_string(),
                 kms_api_key: "test-kms-key".to_string(),
+                api_key_ssm_arn: String::default(),
                 serverless_logs_enabled: false,
                 serverless_flush_strategy: FlushStrategy::Periodically(PeriodicStrategy {
                     interval: 60000,

mod.rs

@@ -337,6 +337,7 @@ pub struct Config {
     // AWS Lambda
     pub api_key_secret_arn: String,
     pub kms_api_key: String,
+    pub api_key_ssm_arn: String,
     pub serverless_logs_enabled: bool,
     pub serverless_flush_strategy: FlushStrategy,
     pub enhanced_metrics: bool,
@@ -440,6 +441,7 @@ impl Default for Config {
             // AWS Lambda
             api_key_secret_arn: String::default(),
             kms_api_key: String::default(),
+            api_key_ssm_arn: String::default(),
             serverless_logs_enabled: true,
             serverless_flush_strategy: FlushStrategy::Default,
             enhanced_metrics: true,

yaml.rs

@@ -972,6 +972,7 @@ api_security_sample_delay: 60 # Seconds
                 api_key_secret_arn: "arn:aws:secretsmanager:region:account:secret:datadog-api-key"
                     .to_string(),
                 kms_api_key: "test-kms-key".to_string(),
+                api_key_ssm_arn: String::default(),
                 serverless_logs_enabled: false,
                 serverless_flush_strategy: FlushStrategy::Periodically(PeriodicStrategy {
                     interval: 60000,