feat: Support env replacement for tool.yaml (#462)

Environment variable replacement is needed so that users don't have to
hardcode their secrets in configuring `tools.yaml`.
Both formats `$ENV_NAME` and `${ENV_NAME}` are standard ways to declare
an environment variable.
However, some database statement placeholders that are already using the
`$ENV_NAME` format.
Therefore, we only support env var declaration using `${ENV_NAME}` to
disambiguate it from other usages.

Fixes issue: https://github.com/googleapis/genai-toolbox/issues/431

docs/en/resources/sources/mssql.md

@@ -7,7 +7,7 @@ description: >
 
 ---
 
-##  About
+## About
 
 [SQL Server][mssql-docs] is a relational database management system (RDBMS)
 developed by Microsoft that allows users to store, retrieve, and manage large
@@ -20,7 +20,7 @@ amount of data through a structured format.
 ### Database User
 
 This source only uses standard authentication. You will need to [create a
-SQL Server user][mssql-users] to login to the database with. 
+SQL Server user][mssql-users] to login to the database with.
 
 [mssql-users]: https://learn.microsoft.com/en-us/sql/relational-databases/security/authentication-access/create-a-database-user?view=sql-server-ver16
 
@@ -29,14 +29,19 @@ SQL Server user][mssql-users] to login to the database with.
 ```yaml
 sources:
     my-mssql-source:
-        kind: "mssql"
-        host: "127.0.0.1"
-        port: "1433"
-        database: "my_db"
-        user: "my-user"
-        password: "my-password"
+        kind: mssql
+        host: 127.0.0.1
+        port: 1433
+        database: my_db
+        user: ${USER_NAME}
+        password: ${PASSWORD}
 ```
 
+{{< notice tip >}}
+Use environment variable replacement with the format ${ENV_NAME}
+instead of hardcoding your secrets into the configuration file.
+{{< /notice >}}
+
 ## Reference
 
 | **field** | **type** | **required** | **description**                                                        |