Zed service option to disable "write" operations

[philrz]

At the time of the filing of this issue, Zed is at commit f733ef3.

Whereas operators in the Zed language to date have been "read-only", the recently-added load operator (#2765) or the planned delete operator (#4142) are unique in that they allow changes to be written back to the pool. For the bulk of current users that are running standalone desktop environments this is probably not of much concern since the same user could easily drop to the shell and run CLI commands like zed load and zed delete to make the equivalent changes to the pool. However, https://github.com/brimdata/zed/issues/2765#issuecomment-1505994586 described how a particular community zync user has an environment where they want to offer read-only query access to a subset of their users and hence are concerned that operators like load and delete could be executed by those users and corrupt storage.

Since Zed currently lacks RBAC (#4892) and such to allow fine-grained control over certain operations, one idea that's been proposed to help in the short term would be to have some kind of flag for starting the Zed service that could disable any "write" operations such as load and delete.