Use base64 encoded string for sse-c key

roles/custom/matrix-synapse/templates/synapse/ext/s3-storage-provider/env.j2

@@ -6,7 +6,7 @@ ENDPOINT={{ matrix_synapse_ext_synapse_s3_storage_provider_config_endpoint_url }
 BUCKET={{ matrix_synapse_ext_synapse_s3_storage_provider_config_bucket }}
 
 {% if matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_enabled %}
-SSE_CUSTOMER_KEY={{ matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_key }}
+SSE_CUSTOMER_KEY={{ matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_key | b64decode }}
 SSE_CUSTOMER_ALGO={{ matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_algo }}
 {% endif %}
 

roles/custom/matrix-synapse/templates/synapse/ext/s3-storage-provider/media_storage_provider.yaml.j2

@@ -10,7 +10,7 @@ config:
   secret_access_key: {{ matrix_synapse_ext_synapse_s3_storage_provider_config_secret_access_key | to_json }}
 
 {% if matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_enabled %}
-  sse_customer_key: {{ matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_key | to_json }}
+  sse_customer_key: {{ matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_key | b64decode | to_json }}
   sse_customer_algo: {{ matrix_synapse_ext_synapse_s3_storage_provider_config_sse_customer_algo | to_json }}
 
 {% endif %}